Compression ignition diesel engines have great utility and advantage as vehicle power plants because of their inherent high thermal efficiency (i.e. good fuel economy) and high torque at low speed. Diesel engines run at a high A/F (air to fuel) ratio under very lean fuel conditions. Because of this, diesel engines have very low emissions of gas phase hydrocarbons and carbon monoxide. However, diesel exhaust is characterized by relatively high emissions of nitrogen oxides (NOx) and particulates. The particulate missions, which are measured as condensed material at 52° C., are multi-phase being comprised of solid (insoluble) carbon soot particles, liquid hydrocarbons in the form of lube oil and unburned fuel, the so called soluble organic fraction (SOF), and the so called “sulfate” in the form of SO3+H2O═H2SO4.
From the standpoint of emissions, however, diesel engines present problems more severe than their spark-ignition counterparts. Emission problems relate to particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others. NO is of concern because it is believed to undergo a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons, and is significant contributor to acid rain. NO2 on the other hand has a high potential as an oxidant and is a strong lung irritant. Particulates (PM) are also connected to respiratory problems. As engine operation modifications are made to reduce particulates and unburned hydrocarbons on diesel engines, the NO2 emissions tend to increase.
The two major components of particulate matter are the volatile organic fraction (VOF) and a soot fraction (soot). The VOF condenses on the soot in layers, and is derived from the diesel fuel and oil. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the exhaust gas. Soot is predominately composed of particles of carbon. The particulate matter from diesel exhaust is highly respirable due to its fine particle size, which poses health risks at higher exposure levels. Moreover, the VOF contains polycyclic aromatic hydrocarbons, some of which are suspected carcinogens.
Oxidation catalysts that contain platinum group metals, base metals and combinations thereof are known to facilitate the treatment of diesel engine exhaust by promoting the conversion of both HC and CO gaseous pollutants and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOCs), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. In addition to the conversions of gaseous HC, CO and particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
The soot, on the other hand, is conventionally reduced by the incorporation of a soot filter in the diesel engine exhaust system. The soot filter is composed of wire mesh, or more commonly a porous ceramic structure. As the soot is trapped in the filter, however, back pressure in the exhaust system increases. One strategy for relieving this backpressure is to combust the soot deposited on the filter, thus unclogging the filter. Some soot filters incorporate catalysts specifically for the combustion of the soot (soot combustion catalysts). The temperatures at which soot combusts with air (containing O2), however, is in excess of 500° C., which may be damaging to the soot filter depending on the accumulated soot.
A filter known in the art for trapping particulate matter is a wall-flow filter. Such wall-flow filters can comprise catalysts on the filter and burn off filtered particulate matter. A common construction is a multi-channel honeycomb structure having the ends of alternate channels on the upstream and downstream sides of the honeycomb structure plugged. This results in checkerboard type pattern on either end. Channels plugged on the upstream or inlet end are opened on the downstream or outlet end. This permits the gas to enter the open upstream channels, flow through the porous walls and exit through the channels having open downstream ends. The gas to be treated passes into the catalytic structure through the open upstream end of a channel and is prevented from exiting by the plugged downstream end of the same channel. The gas pressure forces the gas through the porous structural walls into channels closed at the upstream end and opened at the downstream end. Such structures are primarily known to filter particles out of the exhaust gas stream. Often the structures have catalysts on the substrate, which enhance the oxidation of the particles. Typical patents disclosing such catalytic structures include U.S. Pat. Nos. 3,904,551; 4,329,162; 4,340,403; 4,364,760; 4,403,008; 4,519,820; 4,559,193; and 4,563,414.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particulates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water.
U.S. Pat. No. 4,510,265 describes a self-cleaning diesel exhaust particulate filter, which contains a catalyst mixture of a platinum group metal and silver vanadate, the presence of which is disclosed to lower the temperature at which ignition and incineration of the particulate matter is initiated. Filters are disclosed to include thin porous walled honeycombs (monoliths) or foamed structures through which the exhaust gases pass with a minimum pressure drop. Useful filters are disclosed to be made from ceramics, generally crystalline, glass ceramics, glasses, metals, cements, resins or organic polymers, papers, textile fabrics and combinations thereof.
U.S. Pat. No. 5,100,632 also describes a catalyzed diesel exhaust particulate filter and a method of removing deposits from the exhaust gas of a diesel engine. The method involves passing the exhaust gases through a catalyzed filter having porous walls where the walls have thereon as a catalyst a mixture of a platinum group metal and an alkaline earth metal. The catalyst mixture is described as serving to lower the temperature at which ignition of collected particulate matter is initiated.
U.S. Pat. No. 4,902,487 is directed to a process wherein diesel exhaust gas is passed through a filter to remove particulate therefrom before discharge. Particulate deposited on the filter is combusted. According to the disclosure the particulate is combusted with a gas containing NO2. It is disclosed that the NO2 is catalytically generated in the exhaust gas before it is passed downstream to the filter on which diesel particulate is entrapped. The NO2 oxidant serves to effectively combust the collected particulate at low temperature and thus reduce the back pressure normally caused by particulate disposition on the filter. It is disclosed that, there should be enough NO2 in the gas fed to the filter to effectively combust the deposited carbon soot and like particulates. Catalysts known to form NO2 from NO are disclosed to be useful. Such catalysts are disclosed to include platinum group metals such as Pt, Pd, Ru, Rh or combinations thereof, and platinum group metal oxides. The downstream filter can be any conventional filter. In a specific embodiment, a ceramic honeycomb monolith is coated with an alumina washcoat carrying a Pt catalyst. A particulate filter is downstream of the monolith. Carbonaceous particulate is disclosed to be combusted at a temperature generally in the order of 375° C. to 500° C. EPO 835 684 A2 discloses a system wherein the upstream catalyst is followed by a downsteam catalyzed flow-through monolith. Although U.S. Pat. No. 4,902,487 discloses benefits to making NO2, U.S. Pat. No. 5,157,007 teaches suppressing NO2 due to the fact that the toxicity of NO2 exceeds that of NO.
U.S. Pat. No. 4,714,694 discloses alumina stabilized ceria catalyst compositions. There is disclosed a method of making a material which includes impregnating bulk ceria or a bulk ceria precursor with an aluminum compound and calcining the impregnated ceria to provide an aluminum stabilized ceria. The composition further comprises one or more platinum group catalytic components dispersed thereon. The use of bulk ceria as a catalyst support for platinum group metal catalysts other than rhodium, is also disclosed in U.S. Pat. No. 4,727,052 of C. Z. Wan, et al. and in U.S. Pat. No. 4,708,946 of Ohata, et al.
U.S. Pat. No. 5,597,771 discloses the use of ceria in catalyst compositions both in bulk form, as a particulate material, and in intimate contact with the various components of the catalyst composition. The intimate contact can be accomplished by combining a ceria containing component with at least some of the other components as a soluble cerium salt. Upon application of heat, such as by calcining the cerium salt becomes ceria.
U.S. Pat. Nos. 4,624,940 and 5,057,483 refer to ceria-zirconia containing particles. It is found that ceria can be dispersed homogeneously throughout the zirconia matrix up to 30 weight percent of the total weight of the ceria-zirconia composite to form a solid solution. A co-formed (e.g., co-precipitated) ceria oxide-zirconia particulate composite can enhance the ceria utility in particles containing ceria-zirconia mixture. The ceria provides the zirconia stabilization and also acts as an oxygen storage component. The '483 patent discloses that neodymium and/or yttrium can be added to the ceria-zirconia composite to modify the resultant oxide properties as desired.
U.S. Pat. No. 5,491,120 discloses oxidation catalysts containing ceria and a bulk second metal oxide which may be one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and alpha-alumina.
U.S. Pat. No. 5,627,124 discloses oxidation catalysts containing ceria and alumina. It is disclosed that each have a surface area of at least about 10 m2/g. The weight ratio of ceria to alumina is disclosed to be 1.5:1 to 1:1.5. It is further disclosed to optionally include platinum. The alumina is disclosed to preferably be activated alumina. U.S. Pat. No. 5,491,120 discloses oxidation catalysts containing ceria and a bulk second metal oxide, which may be one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and alpha-alumina.
The prior art also shows an awareness of the use of zeolites, including acidic zeolites and metal-doped zeolites, to treat diesel exhaust. European Patent 0 499 931 B 1 is directed to the use of a catalyst for reducing the quantity and/or size of particles and exhaust gases of diesel engines. This catalyst is characterized in using zeolites such as faujasite, pentasil or mordenite with acidic properties to crack to long chain and aromatic hydrocarbons. This patent claims priority from German Patent DE 4105534C2, which discloses the use of acidic zeolites to crack long chain hydrocarbons. Additionally, DE 4226111A1 and DE 4226112A1 are patents which also disclose the use of acidic zeolites. In DE 4226111A1, noble metal and acid zeolites are disclosed as a composition to catalyze the reduction of mass and/or size of particles. DE 4226112A1 discloses compositions using transitional metal oxide and an acid zeolite for similar reasons. U.S. Pat. No. 5,330,945 discloses a catalyst treatment of diesel exhaust particles. Such a composition is includes a zeolite having exchangeable cations at cationic sites in combination with silica and very fine particles of catalytic metal. The goal, here again, is to permit penetration of hydrocarbons to be cracked and oxidized.
WO 94/22564 discloses a catalyst composition for treating diesel exhaust which includes ceria and optionally alumina as well as a Beta zeolite. A platinum group metal is employed to promote oxidation of CO and HC while limiting the conversion of SO2 to SO3.
WO 94/01926 entitled, “Improved Zeolite-Containing Oxidation Catalyst and Method of Use” discloses catalyst compositions for treating a diesel engine exhaust stream containing a volatile organic fraction. A catalyst composition comprises a refractory carrier on which is disposed a coating of a catalytic material comprising a catalytically effective amount of ceria having a BET surface area of at least about 10 m2/g and a catalytically effective amount of a zeolite. It is also known to employ ceria and alumina as a support for a platinum group metal as a dual exhaust catalyst. The zeolite can be doped with a platinum group metal. In this composition the zeolite is employed to serve both to catalyze the oxidation of VOF and to crack the larger VOF molecules and, during period of relatively low temperature operation, to trap gas-phase hydrocarbons within the zeolite pores. If the zeolite has been doped with one or more catalytic metals or hydrogen, the trapped gas-phase hydrocarbons are brought into intimate contact with the catalytically active cations, which facilitates oxidation of the hydrocarbons.
To reach ever higher standards of emission control, original equipment manufacturers (OEMs) typically use one or more diesel oxidation catalysts (DOC) in front of a catalyzed soot filter (CSF) to bring vehicles into compliance. Specifically, DOCs in a close-coupled (CC) position to rapidly light-off (burn) the filtered particulate matter have been used. This is a very costly approach. Many OEMs have suggested CC CSFs alone or in combination with a very thin slice of a DOC to help meet emissions standards and reduce costs. However, to apply this approach the CSF must have the full catalytic capabilities of a DOC.
Typically, low porosity wall-flow substrates for use as CSFs limit washcoat layers to below 0.5 g/in3 due to backpressure constraints. Higher porosity (>50%) substrates can be used to carry increased washcoat loadings necessary for the CSF to have the full catalytic capabilities of a DOC. However, these higher porosity substrates are difficult and costly to produce and are still limited by the amount of traditional washcoat (typically 1 g/in3) that can be applied and still meet backpressure requirements. When one considers that sufficient DOC catalyst washcoats can exceed 2 g/in3, it becomes apparent that there is a dichotomy between the level of washcoating needed to provide sufficient DOC performance and backpressure constraints. Therefore, it is an object of the present invention to provide a washcoat layer, which when applied to a catalyzed soot filter in sufficient quantity to provide the catalytic capabilities of a diesel oxidation catalyst will not cause excessive back pressure across the coated article when implemented in an emission treatment system.